It is known that the ratio between the flow of gas to be burned and the air flow required for combustion should at all times be equal to or greater than the stoichiometric ratio in order for a burner to be able to operate entirely independently with no additional supply of external air. It is also known that the combustion of gas containing hydrocarbons, to burn off waste gases containing hydrogen sulfide, for example, or gas emissions containing hydrocarbons at an oil refinery or oil or gas production field, must meet three essential conditions, generally called “the three T's” in the industry, to obtain optimal combustion.
Indeed, if the quantity of air required for combustion is insufficient and if these three T's, that is, the Temperature of the flame, the Time the air and gas are mixed before burning, and the Turbulence applied to this mixture, are not observed, foul smelling odors and thick black smoke, essentially composed of unburned hydrocarbons, are emitted by the incomplete combustion of these gases or gaseous emissions, thus harming the environment.
One of the major causes of this incomplete combustion, and particularly of the appearance of black smoke at the point of combustion, is therefore primarily the result of an insufficient amount of air, detrimental to good combustion free of unburned hydrocarbons. Indeed, when the flow of gas to be burned is large, for example, and the supply pressure of this fuel gas is very low, commercially available burners are generally ineffective in permitting aeration of the flame sufficiently activated by the pressure of the gas coming out of the supply tube of the burner. An external supply of air needed for combustion must therefore be provided, by any means available in the technology, for example by supplying a fuel-oxidant mixture energy by means of water vapor.
Devices have been proposed to obtain sufficient mixing energy from large quantities of air, using external fluids such as water vapor going through injectors, or other fluids acting as “motive fluids,” for example when the air itself is compressed, or using powerful blowers to induce the air and turbulence required for combustion. These devices generally have low yields, so in order to compensate for their lack of efficiency it is necessary to use large quantities of motive fluids which are not always available in the amounts required on the waste gas combustion site.
As a result, when water vapor is used as a motive fluid, for example, the high rate of consumption produces the following disadvantages:                high noise emission due to the passage of the fluid in the tubes and injectors,        a cooling of the flame so that the correct conditions for combustion of the gases are not ensured; for example, acidic gases such as H2S, for which the temperature of 700° C. required for complete oxidation is not reached under these conditions, resulting in toxic and foul smelling emissions.        an energy balance on the site could be deficient because of the dependence on production of water vapor.        
In oil and gas production fields, water vapor is generally not available and because the pressure of gas to be burned is too low to act as motive gas, that is, that the gas itself could entrain enough air needed for its combustion and thus obtain sufficient fuel-oxidant mixing energy, the combustion of these gases containing hydrocarbons is therefore incomplete, resulting in thick black smoke from the burning site.
Manufacturers of burners have proposed a system of aeration of the flame of a burner that consists of supplying air for the combustion by means of high power blower units arranged beneath the burner, and by using automatic valves to control the distribution of gas in accordance with the capacity to be burned. There is an unacceptable risk of failure in this system, controlled by complex instrumentation, because a dangerous obstruction could be created in the manifold of gases to be burned at a refinery, when for example an automatically controlled valve remains in the closed position. Furthermore, this system for aeration of the flame proves to have a high capital cost and operating expenses even while it is not very reliable and generates safety problems when the gas to be burned contains liquid hydrocarbons, also called condensates, easily flammable, which can fall on the blowers arranged beneath the burner.
FR-A-2 095 661 deals with an air aspirator using as a motive fluid a gas under pressure, the nature of which is not specified. This air aspirator has a plurality of gas injection tubes arranged in two concentric rings at the input of a venturi tube open to the atmosphere and the axes of which are parallel to the axis of the venturi tube.
U.S. Pat. No. 2,403,431 describes a fuel gas burner having a plurality of injection tubes arranged at the input of a venturi tube open to the atmosphere. These tubes, parallel to the axis of the venturi tube, are supplied with a premixture of fuel gas and air, this premixture coming from another venturi tube placed upstream and open to the atmosphere or connected to a source of air under pressure.
European patent No. 99 828, held by the Applicant, proposes a device for the combustion of mixtures of fuel fluids with air induction, in which these fluids are introduced into a burner body forming a venturi, by means of injectors with annular cross section arranged coaxially along the axis of said body. Such a device has the major disadvantage of being difficult to produce because it requires extremely careful machining. Moreover, it seemed desirable to improve its performance in order to improve the efficiency of the device and particularly the homogeneity of the fluid ejection velocity profile in the mixing tube of the venturi forming the burner.
The applicant has therefore done research to find solutions that are technologically satisfactory, simple, reliable, with low capital cost at a refinery as well as at a production site, to provide sufficient quantities of air for burning a gas fed under low pressure and containing hydrocarbons, while improving the conditions required for optimal combustion of this gas and in particular to obtain smokeless combustion.